1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More specifically, the invention relates to support structures for towed marine seismic source arrays.
2. Background Art
Marine seismic surveying is typically performed by towing one or more arrays of seismic energy sources behind a survey vessel. A plurality of seismic sensor cables can also be towed by the survey vessel at laterally spaced apart positions with respect to the center line of the survey vessel.
The source arrays are coupled to the survey vessel by an “umbilical cable” which provides axial strength to tow the source arrays, electrical signal conductors to actuate the individual sources and, when the seismic sources are air guns, compressed air lines to charge the guns between actuations. An array of sources is typically used in order to provide greater seismic signal bandwidth, by selecting individual source having different sizes and thus different energy frequency output.
Two different types of source arrays known in the art will be explained to illustrate some of the sources of failure of array support structures known in the art. In FIG. 1, an array structure characterized as a rigid (float) array is shown. A float 10 with an attached keel 11, typically using band clamps 12 or similar device, moves along the surface of the water. The umbilical cable 24 is shown attached to the front and aft of the keel. The aft end of the umbilical cable is terminated in a steel housing with connectors for electric and optical signals, and connector for air pressure. The cables and hoses connected to the termination connectors forms into a tail loop, which then extends forward under the water surface. A harness pipe 14 may be suspended at a selected depth in the water using depth ropes 20. The harness pipe 14 also supports an air manifold 16. The manifold 16 may be suspended from the harness pipe using clamps 18. An air gun 22 is suspended by wires or chains from each clamp 18. The air pressure is fed into the manifold 16 and each air gun 22 is supplied though air outlets welded to the manifold 16. Electrical signals, or other signals, are distributed to and from the air guns 22, and other equipment that may be mounted on the array, through cut-outs in the harness pipe 14.
To absorb some of the flexural motion that inevitably is applied to the gun array, the harness pipe 14 and the manifold 16 include flexural sections F. The flexural sections F include a tow wire to transmit axial loading and a bend restrictor to stiffen/control the flexibility, and the harness pipe 14 and manifold 16 are connected across the flexural sections F using hose or similar flexible conduit. One of the difficulties with the foregoing arrangement is bend fatigue at the connections between the flexural sections F and the adjacent stiff section components, and fatigue in the stiff section components themselves.
Another source array known in the art is shown in FIG. 2. The array shown in FIG. 2 is typically characterized as a flexible (float) array. The umbilical cable 24 is connected to a forwardmost harness pipe 25. Each harness pipe 25 may be suspended at a selected depth in the water by cables or ropes 20 coupled to associated float segments 10A through 10G. A flexible hose 21 connects the harness pipes 25 to one another. A rigid air manifold 10 is suspended beneath the harness pipe 25 by means of the clamps 26. A flexible hose 23 connects the air manifolds to one another. Air guns 22 may be suspended from the short sections of air manifold using chains or cables. Axial loading is typically transferred across the flexible joints using a tow wire, chain or cable 27. The arrangement in FIG. 2 is also susceptible to bending fatigue and failure. Further, in order to have available both of the array arrangements shown in FIG. 1 and FIG. 2, it is necessary to keep complete inventories of both types of rigid sections (“gun boxes”) and flexural members.
There exists a need for improved seismic source array support structures.